Remote Radio Heads (RRHs) are used to extend coverage of a base station. As part of the work on carrier aggregation, next-generation cellular communication systems will support carrier aggregation of frequencies on which RRHs are deployed. Carrier Aggregation will be used to provide improved data rates to users. Carrier aggregation consists of transmitting data to or receiving data from the user equipment (UE) on multiple carrier frequencies (“component carriers”). The wider bandwidth enables higher data rates.
A UE can be configured with a set of component carrier (CCs). Specifically, the UE is configured with a cell on each component carrier. Some of these cells may be activated. The activated cells can be used to send and receive data (i.e., the activated cells can be used for scheduling). When there is a need for aggregating multiple CCs (e.g., a large burst of data), the network can activate configured cells on one or more of the CCs. There is a designated primary cell (Pcell) on a CC that is referred to as the primary CC, which is always activated. The other configured cells are referred to as SCells (and the corresponding CCs are referred to as secondary CCs).
RRHs are deployed on a different frequency than the frequency used by the base station site and provide hot-spot like coverage on that frequency. User equipment (UE) that is in such a hot-spot can perform carrier aggregation of the frequency used by the base station and the frequency used by the RRH and obtain corresponding throughput benefits. RRHs do not embody typical base station functionalities such as higher layer processing, scheduling etc. The baseband signal transmitted by an RRH is generated by the base station and is carried to the RRH by a high speed wired (e.g., optical) link. Thus RRHs function as remote antenna units of a base station, with a high speed link to the base station.
A base station 101, RRH 102, and UE 103 are shown in FIG. 1. As is evident, a non-wireless link 104 exists between base station 101 and RRH 102. The transmissions to UE 102 occur both from base station 101 and from RRH 102, except that the transmissions from base station 101 exist on a different frequency than the transmissions from RRH 102.
The presence of RRHs introduces additional physical locations from which the UE can receive the base station signal (i.e., in addition to receiving the base station signal directly from the base station). In addition, there is a delay introduced by the communication between the base station and the RRH. This delay results in the UE perceiving very different propagation delays on the frequency used by the base station and the frequency used by the RRH. As a consequence, the timing advance applied to the two frequencies need to be different.
The base station assigns a timing advance to a UE to ensure that the uplink transmission by the UE is received at the base station at the same time as the downlink transmission is started (reception of uplink subframe boundary is aligned to transmission of downlink subframe boundary). Furthermore, the base station ensures that uplink transmissions by different UEs are received at the same time, by assigning different UEs different timing advances (based on the propagation delay).
For carrier aggregation with a single timing advance (i.e., same timing advance is used for all participating uplink CCs), the uplink transmissions are time aligned. However, in the presence of RRHs this assumption is no longer valid.
FIG. 2 shows the timing relationships between downlink and uplink transmissions of the two frequencies. In particular, downlink (DL) transmission (Tx) is shown on frequency 1 (F1) as subframe 201, DL reception (Rx) is shown on F1 as subframe 202, UL Tx is shown on F1 as subframe 203, UL Rx is shown on F1 as subframe 204. In a similar manner DL Tx is shown on F2 as subframe 205, DL Rx is shown on F2 as subframe 206, UL Tx is shown on F2 as subframe 207, and UL Rx is shown on F2 as subframe 208.
It is assumed that base station 101 tries to ensure that uplink transmissions on F1 and F2 are received at the same time. Transmissions on F2 through RRH 102 (both uplink and downlink) have an additional delay due to transmission through fiber link 104 and the associated RRH processing. This additional delay can be as large as 30 microseconds. As shown in FIG. 2, in order for the F2 uplink to arrive at the base station at the same time as the F1 uplink, the timing advance applied by the UE for transmissions on F2 has to compensate for the fiber and RRH processing delay.
As a result, the uplink subframes 203, 204,207, and 208 on F1 and F2 are not time aligned. In FIG. 2, F2 uplink subframe 207 starts before F1 uplink subframe 203. Specifically, the last symbol of uplink subframe n−1 on F1 overlaps the first symbol of uplink subframe n on F2. If a UE is required to transmit in both subframe n−1 on F1 and subframe n on F2, the power available to transmit the last symbol of subframe n−1 on F1 or the first symbol of subframe n on F2 may be limited. For example, if the UE is at the edge of the macro cell, the UE may not be able to transmit at the required power since the power required during the overlapping symbol may exceed the SAR limit. Therefore a need exists for a method and apparatus for power allocation for overlapping transmissions when multiple timing advances are used that reduces a number of times a mobile unit may be unable to transmit because its transmissions exceed the SAR limit.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. Those skilled in the art will further recognize that references to specific implementation embodiments such as “circuitry” may equally be accomplished via either on general purpose computing apparatus (e.g., CPU) or specialized processing apparatus (e.g., DSP) executing software instructions stored in non-transitory computer-readable memory. It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.